Control of a personal transporter based on user position

ABSTRACT

An apparatus controller for prompting a rider to be positioned on a vehicle in such a manner as to reduce lateral instability due to lateral acceleration of the vehicle. The apparatus has an input for receiving specification from the rider of a desired direction of travel, and indicating means for reflecting to the rider a propitious instantaneous body orientation to enhance stability in the face of lateral acceleration. The indicating may include a handlebar that is pivotable with respect to the vehicle and that is driven in response to vehicle turning.

The present application is a continuation of U.S. application Ser. No.12/879,650, filed Sep. 10, 2010, which is a continuation of U.S.application Ser. No. 11/863,640, filed Sep. 28, 2007, now U.S. Pat. No.7,812,715, which is a divisional of U.S. application Ser. No.10/939,955, filed Sep. 13, 2004, now U.S. Pat. No. 7,275,607, which is acontinuation-in-part of U.S. application Ser. No. 10/308,850, filed Dec.3, 2002, now U.S. Pat. No. 6,789,640. U.S. application Ser. No.10/308,850, in turn, claimed priority from U.S. Provisional ApplicationNo. 60/388,846, filed Jun. 14, 2002. U.S. application Ser. No.10/939,955 was also a continuation-in-part of U.S. application Ser. No.10/044,590, filed Jan. 11, 2002, now abandoned, which was a divisionalof U.S. application Ser. No. 09/635,936, filed Aug. 10, 2000, now U.S.Pat. No. 6,367,817, which was a divisional of U.S. application Ser. No.09/325,978, filed Jun. 4, 1999, now U.S. Pat. No. 6,302,230. The presentapplication claims priority from all of the foregoing applications byvirtue of the priority chain heretofore recited. All of the foregoingapplications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention pertains to control of personal transporters, andmore particularly to devices and methods for providing user input withrespect to either directional or velocity control of such transporters(having any number of ground-contacting elements) based on the positionor orientation of a user.

BACKGROUND OF THE INVENTION

Dynamically stabilized transporters refer to personal transportershaving a control system that actively maintains the stability of thetransporter while the transporter is operating. The control systemmaintains the stability of the transporter by continuously sensing theorientation of the transporter, determining the corrective action tomaintain stability, and commanding the wheel motors to make thecorrective action.

For vehicles that maintain a stable footprint, coupling between steeringcontrol, on the one hand, and control of the forward motion of thevehicles is not an issue of concern since, under typical roadconditions, stability is maintained by virtue of the wheels being incontact with the ground throughout the course of a turn. In a balancingtransporter, however, any torque applied to one or more wheels affectsthe stability of the transporter. Coupling between steering andbalancing control mechanisms is one subject of U.S. Pat. No. 6,789,640,which is incorporated herein by reference. Directional inputs thatadvantageously provide intuitive and natural integration of humancontrol with the steering requirements of a balancing vehicle are thesubject of the present invention.

SUMMARY OF THE INVENTION

In accordance with preferred embodiments of the present invention, acontroller is provided that may be employed for providing user input ofa desired direction of motion or orientation for a transporter. Thecontroller has an input for receiving specification by a user of a valuebased on a detected body orientation of the user.

User-specified input may be conveyed by the user using any of a largevariety of input modalities, including: ultrasonic body positionsensing; foot force sensing; handlebar lean; active handlebar;mechanical sensing of body position; and linear slide directional input.

In those embodiments of the invention wherein the transporter is capableof balanced operation on one or more ground-contacting elements, aninput is provided for receiving specification from the user of a desireddirection of motion, or a desired velocity value based on a detectedbody orientation of the user. A processor generates a command signalbased at least on the user-specified direction and velocity value inconjunction with a pitch command signal that is based on a pitch errorin such a manner as to maintain balance of the transporter in the courseof achieving the specified direction and velocity. The input of adesired direction may also include a user-specified yaw value, yaw ratevalue, or fore/aft direction.

In various other embodiments of the invention, the controller has asummer for differencing an instantaneous yaw value from theuser-specified yaw value to generate a yaw error value such that the yawcommand signal generated by the processor is based at least in part onthe yaw error value. The input for receiving user specification mayinclude a pressure sensor disposed to detect orientation of the user, anultrasonic sensor disposed to detect orientation of the user, or a forcesensor disposed on a platform supporting the user for detecting weightdistribution of the user. In yet other embodiments, the input forreceiving user specification includes a shaft disposed in a planetransverse to an axis characterizing rotation of the two laterallydisposed wheels, the desired direction and velocity specified on thebasis of orientation of the shaft.

In accordance with further embodiments of the invention, the balancingtransporter may includes a handlebar, and the controller may furtherhave a powered pivot for positioning the handlebar based at least uponone of lateral acceleration and roll angle of the transporter. Inparticular, the controller may have a position loop for commanding ahandlebar position substantially proportional to the difference in thesquare of the velocity of a first wheel and the square of the velocityof a second wheel.

In accordance with yet other embodiments of the invention, an apparatusis provided for prompting a rider to be positioned on a vehicle in sucha manner as to reduce lateral instability due to lateral acceleration ofthe vehicle. The apparatus has an input for receiving specification bythe rider of a desired direction of travel and an indicating means forreflecting to the rider a desired instantaneous body orientation basedat least on current lateral acceleration of the vehicle. The indicatingmeans may include a handlebar pivotable with respect to the vehicle, thehandlebar driven in response to vehicle turning.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description, taken with referenceto the accompanying drawings, in which:

FIG. 1 shows a personal transporter, as described in detail in U.S. Pat.No. 6,302,230, to which the present invention may advantageously beapplied;

FIG. 2 shows a block diagram showing the constitutive inputs and outputsof a yaw command in a system architecture to which the present inventionmay be advantageously applied;

FIG. 3A is an exploded view of components of a yaw control mechanismshowing a yaw control grip coupled to a user interface of a personaltransporter, in accordance with an embodiment of the present invention;

FIG. 3B shows a detailed exploded view of the yaw control grip of FIG.3A;

FIG. 3C shows the integral yaw control sensor of the yaw controlmechanism of FIG. 3A;

FIG. 4 shows a schematic block diagram of a yaw-feedback control systemin accordance with embodiments of the present invention;

FIG. 5A is a schematic top view of a rider in positions indicating fullsquare positioning, a tilt to the left, and a counterclockwise rotation,respectively;

FIG. 5B is a front view of a hip collar for detecting changes in riderorientation to control yaw in accordance with an embodiment of thepresent invention;

FIG. 5C is a diagram of an ultrasound transmitter/receiver configurationin accordance with various embodiments of the present invention;

FIG. 5D is a waveform timing display of ultrasound signals transmittedand received by components of embodiments of the present inventiondepicted in FIG. 4A;

FIG. 6A is a top view of the platform of a personal transporter with thepressure plate removed, indicating the placement of feet-force pressuresensors in accordance with various embodiments of the present invention;

FIG. 6B is a diagram of a pressure plate for application of force by auser in embodiments of the present invention depicted in FIG. 6A;

FIG. 6C is a schematic depicting the development of a yaw command signalfrom the foot-force sensors of FIG. 6A, in accordance with an embodimentof the present invention;

FIG. 6D shows a deadband in the command as a function of yaw input;

FIG. 6E shows a ramp function for switching yaw command in reverse as afunction of wheel velocity;

FIG. 7A shows a handlebar lean device for control input to a personaltransporter in accordance with embodiments of the present invention;

FIG. 7B shows a handlebar lean device with flexure coupling of thecontrol stalk to the ground-contacting module for control input to apersonal transporter in accordance with embodiments of the presentinvention;

FIG. 7C shows a further handlebar lean device with separated handles forcontrol input to a personal transporter in accordance with embodimentsof the present invention;

FIG. 7D shows a rotating handlebar device for control input to apersonal transporter in accordance with embodiments of the presentinvention

FIG. 7E shows a handlebar lean device for control input to a personaltransporter in accordance with embodiments of the present invention;

FIG. 7F shows a shock absorber and damping adjustment for use with theembodiment of the invention depicted in FIG. 7A;

FIG. 7G is a block schematic of a mixer block for combining yaw inputand roll information in accordance with embodiments of the presentinvention;

FIG. 7H shows a handlebar bearing and détente allowing the rotationaldegree of freedom of the handlebar to be locked in accordance with anembodiment of the present invention;

FIG. 8A shows the response of the active handlebar to a rolldisturbance, in accordance with an embodiment of the present invention;

FIGS. 8B and 8C show front and back views of active handlebar responseduring a high-speed turn, in accordance with an embodiment of thepresent invention;

FIGS. 9A and 9B show the basic mechanical hardware layout of the poweredhandlebar embodiment of FIGS. 8A-8C;

FIG. 10A shows a front view of a knee position sensor for providingsteering input to a personal transporter in accordance with embodimentsof the present invention;

FIG. 10B shows a centering mechanism employed in conjunction with theknee position sensor of FIG. 10A;

FIG. 10C shows hip position sensors for providing user yaw input inaccordance with an embodiment of the present invention;

FIG. 10D shows a torso position sensor for providing user yaw input inaccordance with an embodiment of the present invention; and

FIG. 11 depicts a linear slide footplate mechanism in accordance withyet another embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

A personal transporter may be said to act as ‘balancing’ if it iscapable of operation on one or more wheels but would be unable to standon the wheels but for operation of a control loop governing operation ofthe wheels. A balancing personal transporter lacks static stability butis dynamically balanced. The wheels, or other ground-contactingelements, that provide contact between such a personal transporter andthe ground or other underlying surface, and minimally support thetransporter with respect to tipping during routine operation, arereferred to herein as ‘primary ground-contacting elements.’

FIG. 1 shows a balancing personal transporter, designated generally bynumeral 10, and described in detail in U.S. Pat. No. 6,302,230, as anexample of a device to which the present invention may advantageously beapplied. A subject 8 stands on a support platform 12 and holds a grip 14on a handle 16 attached to the platform 12. A control loop may beprovided so that leaning of the subject results in the application oftorque to wheel 20 about axle 22 by means of a motor drive depictedschematically in FIG. 2, as discussed below, thereby causing anacceleration of the transporter. Transporter 10, however, is staticallyunstable, and, absent operation of the control loop to maintain dynamicstability, transporter 10 will no longer be able to operate in itstypical operating orientation. “Stability” as used in this descriptionand in any appended claims refers to the mechanical condition of anoperating position with respect to which the system will naturallyreturn if the system is perturbed away from the operating position inany respect.

Different numbers of wheels or other ground-contacting members mayadvantageously be used in various embodiments of the invention asparticularly suited to varying applications. Thus, within the scope ofthe present invention, the number of ground-contacting members may beany number equal to, or greater than, one. A personal transporter may besaid to act as ‘balancing’ if it is capable of operation on one or morewheels (or other ground-contacting elements) but would be unable tostand stably on the wheels but for operation of a control loop governingoperation of the wheels. The wheels, or other ground-contactingelements, that provide contact between such a personal transporter andthe ground or other underlying surface, and minimally support thetransporter with respect to tipping during routine operation, may bereferred to herein as ‘primary ground-contacting elements.’ Atransporter such as transporter 10 may advantageously be used as amobile work platform or a recreational vehicle such as a golf cart, oras a delivery vehicle.

The term “lean”, as used herein, refers to the angle with respect to thelocal vertical direction of a line that passes through the center ofmass of the system and the center of rotation of a ground-contactingelement supporting the system above the ground at a given moment. Theterm “system” refers to all mass caused to move due to motion of theground-contacting elements with respect to the surface over which thevehicle is moving.

“Stability” as used in this description and in any appended claimsrefers to the mechanical condition of an operating position with respectto which the system will naturally return if the system is perturbedaway from the operating position in any respect.

One mechanism for providing user input for a yaw control system of apersonal transporter is described in detail in U.S. Pat. No. 6,789,640.As described therein and as shown in FIGS. 3A-3C, a user mounted on thetransporter may provide yaw control input to a yaw controller 502 (shownin FIG. 2) by rotating yaw grip assembly 800, shown in detail in FIG.3B.

FIG. 2 depicts the differencing, in summer 522, of the current yaw valueψ with respect to the desired yaw value ψ_(desired) to obtain thecurrent yaw error ψ_(err). Desired yaw value ψ_(desired) is obtainedfrom a user input, various embodiments of which are described herein.The current value ψ of yaw is derived from various state estimates, suchas the differential wheel velocities, inertial sensing, etc. Derivationof the yaw command from the yaw error is provided by motor controller 72according to various processing algorithms described, for example, inU.S. Pat. No. 6,288,505, and applied to left and right motors 28 and 30,respectively.

With particular reference to FIG. 3A, one embodiment of user interface14 has twin hollow stalks 802, one on either side, either of which mayserve interchangeably to support yaw grip assembly 800. Thus yaw mayadvantageously be controlled by a specified hand (right or left), eitherside of central control shaft 16. Yaw grip assembly 800 comprises a grip804 which is rotated about an axis 806 coaxial with stalks 802. Springdamper 808 provides an opposing force to rotation of yaw grip 804 andreturns yaw grip 804 to the central neutral position. Yaw grip 804contains at least one magnet 810 (two are shown in FIG. 3B, inaccordance with a preferred embodiment), the rotation of which aboutaxis 806 allows the rotational orientation of grip 804 to be sensed bysensor unit 812 (shown in FIG. 3C) which is disposed within protrudingstalk 802. Thus, user interface 14 may be sealed at its ends with fixedyaw grips 814 and the integral sealed nature of the user interface isnot compromised by the yaw control input. Sensor unit 812 may containHall effect sensors which are preferably redundant to ensure fail-safeoperation. Other magnetic sensors may also be employed within the scopeof the present invention.

FIG. 4 shows a block diagram for the yaw feedback control system, inaccordance with one embodiment of the invention. The LateralAccelScalefunction 42 reduces the effect of the yaw input 40 at higher wheelspeeds and at higher centripetal acceleration. Feedback 44, used toregulate the commanded yaw velocity, contains a yaw position term 45 tomaintain yaw position, a velocity squared term 46 that will attempt toregulate the yaw velocity to zero, and a feedforward term 49 used toprovide better yaw command response to the user.

From FIG. 4, it is apparent that the feedforward term 49 must dominatefor rapid maneuvers in order to provide a responsive system. Thevelocity-squared feedback 46 deviates from linear control theory and hasthe effect of providing nonlinear yaw velocity damping.

Several alternatives to a twist grip input device for specifying userdirectional or velocity input are now described.

Body Position Sensing

In accordance with various embodiments of the present invention, adevice which detects the body position of the rider is employed tocontrol fore/aft motion or steering of a transporter. For purposes ofyaw control, in accordance with various embodiments of the invention,sensors detect whether the hips or shoulders of a rider, shownschematically from above in FIG. 5A, are squarely aligned, or aretranslated in a lateral direction 51 or else rotated, such that oneshoulder is thrust in a forward direction 52 while the opposing shoulderis thrust in a backward direction 53. These schemes can be usedindependently or to provide directional imput.

Any method of sensing of body position to control vehicle yaw orfore/aft motion is within the scope of the present invention and of anyappended claims. One embodiment of the invention, described withreference to FIG. 5B, entails mechanical contact with the rider 8. Pads54 are mounted on yoke 55 and contain pressure transducers that transmitsignals to the yaw controller based on changes in sensed position of thehips of the user. Other methods of sensing user position may rely uponoptical or ultrasonic detection, an example of which is now describedwith reference to FIGS. 5C and 5D.

In one embodiment, an ultrasonic beacon is worn by the rider, and anarray of receivers mounted to the machine detect the position of therider. Time of flight information is obtained from transmitter to eachreceiver and used to calculate the lateral position of the user withrespect to the center of the machine. To turn the machine to the right,the user leans to the right, and similarly for turning left. In additionto the intuitive appeal of a mechanism which translates body motion totransporter control, as in the case of fore-aft motion control of apersonal transporter of the sort described in U.S. Pat. No. 6,302,230,the body-control modality also advantageously positions the user'scenter of gravity (CG) correctly for high speed turns.

A body lean system described with reference to FIGS. 5A and 5B consistsof three distinct mechanical components—the transmitter beacon, thereceiver array, and the processing electronics. In one embodiment of theinvention, an ultrasonic(US)/RF transmitter beacon is worn by the rider,and an array of ultrasonic receivers and an RF receiver is mounted belowthe handlebars of the transporter, along with interface electronics.Various ultrasonic transmitters/receivers may be employed, such as thosesupplied by Devantech Ltd. of Norfolk, England. The transmitter beaconis a small piece of Delrin® acetal resin with three ultrasoundtransmitters, at a typical frequency of 40 kHz, mounted at 90 and ±45degrees. This produces a cone of sound of about 160 degrees. The driverelectronics are mounted on a printed circuit board buried behind thetransmitters, and a small RF transmitter is mounted below. A belt clipfrom a wireless phone attaches the transmitter to the user, while poweris supplied by batteries.

The receiver array is a bar with receivers mounted at various locations.The ultrasound receivers are also mounted in small pieces of Delrin®,with the electronics located behind the bar at each location. Toincrease the size of the reception cone, 2 US receivers are used at eachlocation, one mounted facing straight out and the other at 45 degrees tothat. For the outboard sensors, the 45-degree receiver faced inward andfor the inboard receivers, the 45 degree receiver faced outwards. Thismakes it possible to use the two outboard receivers (left and right) forlocation when the rider is at the center of the machine, but as therider moves right, the two right sensors take over, and the same whenthe rider moves to the left.

An ultrasonic rangefinder, such as a Devantech Model SRF-04, may provideboth the transmitter and receiver functions and circuitry, howevermodifications are within the scope of the present invention. The beaconportion consolidates three drivers onto a single board. In addition, themicrocontroller code residing on the transmitter boards allows the boardto transmit continuously.

The board designed to interface to the computer was breadboarded withmicrocontrollers of the same type as the Devantech boards. The functionof this board is to create a square wave that represents the timedifference of arrival between the RF pulse and the ultrasonic pulse(wave goes high when the RF pulse arrives, and low when the US pulsearrives), and correctly interface this wave to the computer circuitry.Since RF travels at the speed of light, and sound at the speed of sound,this scheme results in an accurate ultrasonic time of flight (TOF)signal.

With reference to FIG. 5D, the counter timer board residing in thecomputer chassis receives 4 waveforms representing the 4 TOF's from thetransmitter to the 4 receivers, and using a 400 KHz clock, determinesthe duration of each pulse (in counts of the 400 khz clock). This infois then passed to the algorithms for distance calculation and additionalprocessing.

When a balancing transporter is traveling at any significant speed, therider needs to lean into a commanded turn in order to counteract thecentripetal accelerations due to the turn. This machine uses the bodylocation sensors to turn the machine. When the rider is centered on themachine, no turn input is generated. When the rider leans to the left, aleft turn is commanded, and the same for the right. Thus the turn is notinitiated until the users CG is properly located. In addition, byknowing the users exact CG location (as by positioning the transmitterat the waist of the rider), the system is able to exactly match thewheel speed/turn rate to the angle the user is at, theoretically exactlycanceling out the forces acting on the body. Thus the amount of turn istuned for the CG location of the rider and wheel speed.

In accordance with the invention, time-of flight (TOF) information froman ultrasonic transmitter is transmitted to at least 2 ultrasonicreceivers. Time of flight was calculated from the difference of a RFreceived pulse edge to a US received pulse edge. Since the speed ofsound is substantially a constant (within the operation of the machine),distance can be calculated from its time of flight from transmitter toreceiver. The law of cosines along with the known distances of thereceivers from center and from each other is then used to calculate thelocation of the transmitter in the lateral direction. Because of theredundant receivers, the lateral location is unique, and immune tochanges in height and fore/aft distance from the bar (unless this changeresulted in a loss of line-of sight (LOS).

Feet Force

In accordance with another embodiment of the invention, yaw controlinput is provided to a transporter by sensing the rider's weightdistribution by using force sensors on the foot plate. In order tosteer, the rider leans in the direction of desired turn. Lean, right:turn right; lean left: turn left. Many variations can be derived from abase system containing many force sensors located at the foot plate.

A PCB board provides all signal conditioning for the force sensors. Thesensor signals are output from the PCB board as zero to five volt analogsignals. Spare A/D inputs on the amplifiers are used to read in the 8analog signals and provide an 8-bit count to the software for eachsensor.

The primary variation implemented and tested uses sensors on the leftand sensors on the right sides of the foot plate. When the person leansto the right, the right sensor signals become large, indicating a turnto the right. When the person leans to the left, the left sensor signalsbecome large, indicating a turn to the left. A special foot plate wasconstructed to allow force distribution to be measured at four cornersof a rigid plate.

The resulting system is advantageously very maneuverable at low speedsand a rider may became more proficient at this yaw input than the twistgrip yaw input. There is a tradeoff between the bandwidth of the devicethat enables the system handle disturbances better, and the perceivedresponsiveness of the system.

As a natural movement, when turning on a personal transporter, a usertends to shift weight in the direction of the turn. The reason for thisis the centripetal force generated by turning tends to push the personoff the transporter. The same user movement is required when riding a3-wheel or a 4-wheel all-terrain-vehicle (ATV). As a result, a naturalinput to turn that encourages good user position is to turn right whenthe user shifts their weight to the right. In this configuration, theuser is in the ideal position to make a right turn.

Skiers and skaters tend to push off with their right foot to turn left.The reason for this is they shift their weight distribution from theright to the left by using their feet. On a personal transporter inaccordance with the present invention, users needs to shift their weightfrom right to left not using their feet, but using the handlebars forthis input device. If the sign were reversed to accommodate the skiersand skaters' preference to lean right and turn left, an unstable systemwould result. As the user leans right, the transporter turns left,generating a centripetal force that pushes the user more to right,generating more left turn command.

Referring to FIG. 6A, the force sensing element is located at the end ofthe flexible ribbon in a circle about the size of a dime. With no forceon the sensor, the resistance is around 800 Mohms. With 100 lbs on thesensor, the output is around 200 Mohms. To condition the signal, op-ampswere used to create an active amplifier and an active anti-aliasingfilter. The goal of the amplifier is to generate an output that isproportional to the change in resistance. The goal of the filter is toprevent measurements above 50 Hz from being measured. An invertingamplifier is used since it provides an output proportional to the changein resistance. An inverting lowpass filter is used as an anti-aliasingfilter and to change to the voltage back to 0 to 5 volts. A +/−12 Voltsupply is used to power the op-amps to remain within the linear saferegions of the op-amps.

The sensors are located under a metal foot-plate, one in each corner, asshown in FIG. 6A. A metal plate on top allows the standard rider-detectbuttons to be pressed while providing a hard surface to press againsteach of the 4 sensors. The metal plate is shown in FIG. 6B.

The overall yaw command from the sensor measurements to the yaw commanddelivered to the control system is shown in FIG. 6C.

Since a user often shifts his weight slightly while standing in place, adeadband is added to the sensor processing. As shown in FIG. 6D,deadband 48 provides a region around zero yaw input where slight yawinputs result in no yaw command.

To calculate the yaw command from the sensors, the following equation isused:

${{\overset{.}{\psi}}_{cmd} = {\frac{\left( {{{right}\mspace{14mu} {front}} + {{right}\mspace{14mu} {rear}}} \right)}{2} - \frac{\left( {{{left}\mspace{14mu} {front}} + {{left}\mspace{14mu} {rear}}} \right)}{2}}},{{where}\mspace{31mu} \psi_{cmd}\mspace{14mu} {is}\mspace{14mu} {the}\mspace{14mu} {commanded}\mspace{14mu} {rate}\mspace{14mu} {of}\mspace{14mu} {change}\mspace{14mu} {in}\mspace{14mu} {{yaw}.}}$

Each sensor provides 0 Volts, or 0 counts with no weight on it, and 5Volts, or 255 counts fully loaded. A deadband of around 40 countsprovided smooth enough control with enough room to feel comfortable.Additionally, filters may be employed to filter the command signal, withpassbands typically centered from about 0.5 Hz to about 3 Hz.

When moving in reverse, if the same equations are used to generate theyaw command, the resulting system has positive feedback. When thetransporter performs an “S-turn”, in reverse, if the user leans to theright, the transporter will turn to the left and create a centripetalforce on the user, pushing the user to the right. To solve this issue, a“C-turn” may be implemented. A ramp function is used to reverse the yawcommand when the transporter begins moving in reverse. To keep aconsistent turning motion, when turning in place, the ramp only switchesthe direction of the yaw command when it moves in reverse. FIG. 6E showsthe ramp function for switching yaw command in reverse as a function ofwheel velocity. The rampRev function is used to modify the yaw velocitycommand as follows:

{dot over (ψ)}_(cmd)={dot over (ψ)}_(cmd)+2·rampRevLPF·{dot over(ψ)}_(cmd)

The rampRev signal is lowpass filtered at 5.0 Hz to smooth the effectsof the ramp.

A brake switch, such as brake switch 7 (shown in FIG. 3A) may beconnected to turn the yaw command off when it is pressed. When thebutton is pressed, a yaw command multiplier of 0 is applied, whereas, ifit is released, the yaw command multiplier is 1. A 0.5 Hz Low PassFilter is used to smooth the transitions between on and off.

Handlebar Lean

One of the key properties of a good directional input device is itsability to provide directional input while managing lateralacceleration. High lateral acceleration turns require the user to leaninto the turn to keep from falling off or tipping over the transporter.An optimal directional input device will require the user to have theirbody properly positioned when commanding a directional input. A twistgrip yaw input, such as discussed above with reference to FIG. 3,encourages proper body positioning through the orientation of itsrotation axis and the design of the knob and handle combination. It ispossible, however, to make an uncoordinated input depending on thedriver's technique

Another method of encouraging proper body positioning is to make one ormore handlebars into a joystick. By pivoting the bar near the base ofthe machine, the user can move his or her body at high speeds and merelyhold onto the handlebar and command an input. When properly tuned, theuser's body is already in position to react against the lateralacceleration at the initiation of the turn, reducing the likelihood ofan improperly coordinated turn.

In the handlebar lean machine, the yaw input is proportional to thehandlebar angle with respect to the chassis. Preferably, the pivot axisis mounted as low as practical on the transporter ground-contactingmodule in order to allow the bar motion to follow the users body motionnaturally, since a person leans most stably by pivoting at the ankles.In other words, a low pivot handlebar tracks the body kinematics. Inthis embodiment, the yaw input is converted into a yaw command usingstandard personal transporter algorithms, which apply a fixed gain toyaw input at low speeds, but scale the gain at higher speed to make theyaw input correspond to lateral acceleration instead of yaw rate. Thisworks well with the handlebar lean device, since the desired lean angleis roughly proportional to lateral acceleration. The result is a verynatural input method, where the user “thinks” right or left via leaning,and the machine follows.

{dot over (ψ)}_(cmd) =K(φ_(HB)−φ_(Roll))

-   -   a. where K is a constant;    -   φ_(HB) is the handlebar angle with respect to the platform;    -   φ_(Roll) is the platform lean with respect to gravity    -   {dot over (ψ)}_(cmd) is the yaw command.

Other embodiments of the invention may have an inclined or horizontallymounted pivot handlebar. In machines with inclined pivots, the angle ofthe pivot with respect to the contact patch and surface providedinteresting turning dynamics. Specifically, the axis of rotation mayaffect the dynamics of turning on a slope or on a flat surface.Preferably, the machine has a low horizontal pivot. A horizontal pivotcan easily track the kinematics of the body during a turn.

In accordance with yet other embodiment of the invention, with thedirection of travel as the reference point, the pivoted handlebar may beeither mounted in the front or the rear of the transporter. Theconfiguration of a rear mounted pivot handlebar enables a user to steerthe transporter with other parts of the body such as the knees, inaddition to using a limb coupled to the handlebar. Furthermore, thetransporter may include a feature that disables the lean steer when auser is mounting or dismounting. The feature may be activated when thetransporter determines that a user is partially on/off the platform suchthat the transporter may not turn into or away from the user whilemounting or dismounting.

Of the various mechanisms suited to provide for handlebar lean, a firstis described with reference to FIG. 7A. Motion of handlebar 700 isconstrained to a plane that is substantially transverse to the directionof forward motion of personal transporter 10 by means of parallel linkbars 702 that are pivotally coupled both to platform 12 and to handlebar700. Motion of the handlebar may also be biased to a central positionand/or damped by means of springs 704 or shock absorbers. In analternate embodiment shown in FIG. 7B, handlebar 700 may be coupled toplatform 12 of the transporter 10 by flexure elements 708, againconstraining motion of the handlebar substantially to a plane transverseto the direction of forward motion and allowing tilting of the handlebarin an arc centered upon a virtual pivot at, or near, the plane ofplatform 12. In either of the embodiments of FIGS. 7A and 7B, one ormore sensors 710 detect the position of handlebar 700 or of members 702coupling the handlebar to the rest of the transporter, either withrespect to the vertical or with respect to a direction fixed withrespect to the ground-contacting module. Sensor 710 may be a load cell,for example, disposed along control shaft 16. Furthermore, the springsor shock absorbers coupled to the handlebar may be used to limit theturning rate of the transporter if desired.

Preferably, the motion of the handlebar is not biased to a centralposition. In embodiments where the handlebar is not biased to a centralposition, there is no preloading around the center and thus a user canprecisely and accurately steer the transporter.

In accordance with an embodiment depicted in FIG. 7C, two separatehandlebar segments 720 and 722 may be moved separately, by leaning ofthe user 8, relative to platform 12 of the transporter. In theembodiment shown, the position of each handlebar segment is biased to aspecified ‘neutral’ height within respective sleeves 724 and 726 bymeans of springs, or otherwise. A relative height offset is transmittedto the yaw controller to control turning, as described in connectionwith other user input modalities.

Yet a further embodiment of the invention is depicted in FIG. 7D, whererotation in clockwise and counterclockwise directions 730 and 732 ofhandlebar 700 relative to support stalk 16 is sensed to generate asignal that transmits a user input to yaw controller 502 (shown in FIG.2). A shock absorber 734 is preferably built in to the pivotal couplingof handlebar 700 about shaft 16.

A handlebar lean device in accordance with a further embodiment of theinvention features a pivot mechanism shown in FIG. 7E. Pivot 70 isadjustable in both spring constant and preload, and has a fixed range ofmotion of ±15°. Preferably, the pivot has an unlimited range of motion.The pivot is mounted as low as possible on the ground-contacting modulechassis 26, and the handle 16 is mounted to the rotating portion of themechanism. A pair of shock absorbers 74 may provide additional dampingand stiffness. Shock absorbers 74 are mounted slightly off horizontal tomaximize their perpendicularity to the control shaft 16 throughout therange of motion.

The shocks are adjustable in both spring constant and damping. Thespring constant is adjustable by pressurizing the shock air reservoir.The damping adjustment is made with a knob that varies an orifice sizeinternal to the shock. Shock absorbers 74 are shown in FIG. 7F. Internalto the pivot mechanism is a cam and spring loaded follower. The camcompresses the follower springs, which generates the restoring springforce. To change the spring constant, a different cam is substituted inthe pivot and some cases the number of Belleville springs is changed.The preload is adjusted externally using a screw, which moves a wedge toposition the Belleville spring stack. Various degrees of stiffness maybe provided by interchangeable cams.

With the stiffest cam installed and the shock absorbers at ambientpressure, a preload of approximately 8 lbs. results, as measured at thehandlebar. Approximately 40 pounds of force are required to deflect thehandlebar to its full 15° travel.

One issue that must be addressed in handlebar lean control is the effectof terrain sensitivity. If the machine is driven over obstacles or roughterrain, a roll disturbance is forced on the machine/rider system sincethe resulting change in position of the user may cause an unintended yawinput is put into the system. Yaw control modalities that depend uponthe overall body lean of a standing person are prone to be moresensitive to terrain than, say, yaw control by means of a twist grip.

To combat this roll sensitivity, a roll compensation algorithm may beemployed. In such an algorithm, the yaw input is modified to compensatefor the roll angle of the chassis, making the yaw input the angle of thehandlebar with respect to gravity. Since it is easier for the user tomaintain body position with respect to gravity rather than the platform,this facilitates rejection of roll disturbances.

In accordance with certain embodiments of the invention, a method forreducing terrain sensitivity employs an algorithm for filtering yawinputs based on the roll rate of the chassis. The instantaneous rate ofrolling, referred to as Roll Rate, is readily available from the PitchState Estimator, such as that described, for example, in U.S. Pat. No.6,332,103, which derives the orientation of the transporter based on oneor more gyroscopes, an inclinometer, or combinations of the above. Largeroll transients cause the rider to be accelerated and, if the rolltransients were to be rigidly coupled, through the rider, to the yawcontrol mechanism, they would cause unintended yaw input.

There are two distinct parts of the solution: rejecting terrain whileriding straight and rejecting terrain while turning; the first is aspecial case of the second. While disabling yaw during periods of largeroll rates would solve the problem for motion in a fixed direction, moreinput is required in order to decouple roll from steered motion.

An unknown input is an estimate of the “intended” yaw input from therider, i.e. the intention, say, to ride around in a 20′ circle. Whilethis information is not directly available, it can be usefully inferredfrom the history of the yaw input. Simply low-pass filtering the dataprovides an estimate of yaw input. However, this causes a response delaythat is noticeable to the rider. On the other hand, if low-pass filtereddata are used only when high roll rates are present, the rider is lesslikely to notice the delay. The algorithm, then, in accordance with apreferred embodiment of the invention, employs a mixer, controlled byroll rate, between direct yaw input and a heavily filtered version.

A transfer function models the amount of roll rate that will couple intothe yaw signal. It is a function of various factors, including thedesign of the yaw input, the rider's ability, and how the rider isholding on to the yaw input. By using this mixing method, the transferfunction can largely be ignored or at most minimized through tuning.

The four main tuning points are: How fast the mixer slews to thefiltered version, how fast the mixer slews back, what threshold the mixstarts and ends, and the corner frequency of the low pass filter (LPF)on yaw input. There are limits to the amount of un-commanded yaw thatcan be removed due to setting the mix threshold. By setting it highthere is more un-commanded yaw, by setting it low there are more falsetrips and the rider will begin to notice the time lag on the yaw signal.Setting the LPF frequency also has tradeoffs. If yaw is too heavilyfiltered, then there will be a noticeable delay and a possibility of yawtransients coupling in from the past. Setting it too low reduces theability of the mixer to remove the transients.

Referring now to FIG. 7G, the mixer block is defined as:

yaw command=F*Yaw Input+(1−F)*Yaw Filtered,

where F is the mixer function which is a continuously varying signalbetween 0.0 and 1.0.

In accordance with various further embodiments of the invention,unintended yaw control is reduced while reacting against the shaft toreposition the body of the rider. The center of rotation at thehandlebar may be repositioned, allowing the user to pull laterally onthe bar without causing any displacement. In another embodiment, shownin FIG. 7H, shaft 16 is free to move in two coupled degrees of freedom.The user is able lock the bar by limiting one DOF by engaging gears 78when they are needed to react against the bar. A yaw command may becomprised of an admixture, linear or otherwise, of inputs derived fromtorque of shaft 16 about its axis and from motion of shaft 16 withrespect to the vertical direction.

Alternatively, a force or torque input may be used. A lateral force loadcell allows the user to torque the bar in order to reposition it.Likewise, a torque sensitive bar may be provided to allow the user topull laterally on the bar.

Another issue that must be addressed in handlebar lean control is theeffect of turning while moving backwards or in reverse. As describedsupra, the system may deal with lean turning while moving backward, byswitching the direction of the yaw command, to perform a “S-turn” or a“C-turn”. Preferably, the system performs a “S-Turn”. The system mayfurther compensate for the dynamics of turning while moving backward bydesensitizing the lean steering movement. Desensitizing the leansteering while reversing can advantageously facilitate using the sameequations to generate the yaw command, the resulting system has positivefeedback.

Any of these foregoing embodiments may be combined, within the scope ofthe present invention, with a rotary yaw control input device such asthat depicted in FIGS. 3A-3C. In this arrangement the rotary control isused for low speed yaw, and the lean device would be used to commandlateral acceleration at higher speeds.

Active Handlebar

In accordance with further embodiments of the invention, an activehandlebar system provides for active control of the handlebar angle withrespect to the chassis. The handlebar is mounted on a powered pivot. Thehandlebar is positioned with respect to the chassis based on lateralacceleration and roll angle of the chassis. If the user maintains goodcoupling with the handlebar, the bar provides assistance in positioningtheir body to improve lateral stability. A skilled user leansautomatically with the bar, exerting almost no lateral force. If anunexpected obstacle or turn is made, however, the active bar can provideassistance to even the most experienced operator. This system is alsoparticularly useful on slopes, both while traversing and during turningmaneuvers.

In order to keep the user most stable, the bar should be positionedparallel the resultant vector of lateral acceleration and gravity. Inthe system described here, lateral acceleration was determined onlyusing the wheel velocities, without taking advantage of any otheravailable state estimator information. Lateral acceleration is given bythe equation:

a_(lat)=ωv

-   -   Where ω is the yaw rate and v is the velocity of the        transporter. ω is based on the difference in wheel velocities        (V_(l) and V_(r)) and the wheel track, T.

$\omega = \frac{V_{l} - V_{r}}{T}$

-   -   v is determined by the average wheel velocity:

$v = \frac{V_{l} + V_{r}}{2}$

-   -   Combing these equations gives:

$a_{lat} = {{\frac{\left( {V_{l} - V_{r}} \right)}{T} \cdot \frac{\left( {V_{l} + V_{r}} \right)}{2}} = \frac{V_{l}^{2} - V_{r}^{2}}{2\; T}}$

Since tan(a_(lat))≈a_(lat) for small angles, the bar position fromvertical is proportional to the difference in the square of each wheelspeed. This position must be compensated by adding the roll angle of thechassis, which results in a handlebar position based on the vector sumof lateral acceleration and the acceleration due to gravity.

The operation of the active handlebar is further described as follows.The user commands yaw, such as with the rotary yaw input shown in FIGS.3A-3C. The user may allow the active bar to assist in the user'spositioning by rigidly coupling to the handlebar with his arms, or hecan maintain a softer coupling and use the active bar to provide himwith feedback. In another embodiment, the user preferably commands yawwith the lean of the handlebar as shown in FIGS. 8A-8C. FIGS. 8A-8C showthe handlebar response to roll and turning events. Note, in FIG. 8C, thealignment of the handlebar with the user's legs.

FIGS. 9A and 9B show the basic mechanical hardware layout of a poweredpivot. The powered pivot is made up of a harmonic drive reduction unit92 powered by an electric motor 90. The output of the drive is coupledto the control shaft via an adapter 94. The powered pivot creates atorque between the chassis 12 and the control shaft 16 (shown in FIG.8A), which can be regulated to provide the position control required bythe active handlebar system. A harmonic drive is a very compact highreduction ratio gear set that is efficient and backdriveable. It worksby using an elliptical bearing, called the “wave generator”, to “walk” aslightly smaller flexible gear 96, called the “flex spline” around theinside of a larger rigid gear, called the “circular spline”. Suitableharmonic drives are available from HD Sysytems, Inc. of Hauppauge, N.Y.and are described in the appended pages.

The active handlebar system uses standard algorithms to control thewheels. The handlebar is controlled with a position loop that commands aposition proportional to the difference in the square of the wheelvelocities. Although a theoretical gain can be calculated and convertedto the proper fixed point units, in practice it was determinedempirically.

The position loop is a standard PID loop using motor encoder data forfeedback. The tuning objectives are good ramp tracking, minimum settlingtime, and minimum overshoot. The loop was tuned using a modifiedtriangle wave.

The handlebar controller used the position at startup as the zero(center) position. The user had to position the bar and hold it centeredat startup. Absolute position feedback may be provided to allow the barto self-center.

Some filtering and dead banding are done to the command beforecommanding the motor. In a specific embodiment, the filtering wasultimately needed to smooth out any noise on the wheel speeds and deadbanding was used to keep the bar still when turning in place on slightlyinclined terrains. A 1 Hz first order filtered estimate of lateralacceleration is multiplied by a first gain (typically, on the order of0.001) and roll compensated by adding roll angle multiplied by a secondgain (typically, on the order of 0.15). Afterwards a software induceddead band, and later compensation, of 15% of the max motor positioncommand (typically, 400 counts.) The final result is filtered by a 0.2Hz filter. This filter may be used to round out the knee introduced atthe dead band and to slow down the movement of the handlebar.

Further Mechanical Sensing of Body Position

In accordance with other embodiments of the invention, the position ofthe rider's body, or of one or more parts thereof, may be sensedmechanically as a means to command yaw or fore/aft motion of a personaltransporter. One such embodiment has been described with reference toFIG. 5B. In accordance with another such embodiment, described withreference to FIG. 10A, body sensing is accomplished by a device 910 thattracks the motion of the right knee through a pivot 912 in line with theankle. Pivot 912 is instrumented with a potentiometer 914, withpotentiometer gains adjusted appropriately to the range of motion of theknee. A controller distinguishes between rider motion intended asaccount for input anomalies caused by terrain. The rider commands a yawinput by shifting his body in the direction he would like to turn, as anexperienced rider of a personal transporter would do, shifting hiscenter of gravity towards the inside of the turn to prevent thecentripetal acceleration of the powerbase from pulling his feet fromunder him.

The yaw input device tracks body position by recording the motion of theright knee as it rotates about a longitudinal axis through the rightankle. The rider interacts with the device through a cuff 910 which fitsclosely around the upper shin just below the knee. The cuff isheight-adjustable and padded to allow a snug fit without discomfort. Thecuff is attached via an arm to a pivot ahead of the foot, located suchthat its axis runs longitudinally in relation to the chassis, and inline with the ankle. (Anthropometric data from Dreyfuss Associates' TheMeasure of Man and Woman suggested the ankle pivot should beapproximately 4″ from the baseplate for an average rider wearing shoes).A potentiometer records the angle of the arm in a manner very similar tothe twist-grip yaw input device described above with reference to FIGS.3A-3C.

A mechanical body position yaw input device incorporates a centeringmechanism that is described with reference to FIG. 10B. A centeringmechanism 920 returns the device to neutral (no yaw input) position whenthe rider is not in contact with the mechanism, and provides tactilefeedback to the user as to the location of the neutral position. Preload(adjustable by adding or subtracting washers) was set such that therider needed to exert a force of 1 kg to move the device from center. Atmaximum travel (25° in either direction) the rider experiences a forceof approximately 2 kg.

In addition to the pivot axis on which the potentiometer is located,there is another non-encoded axis at ankle height, perpendicular to thefirst, which allows the cuff to move with the knee as the rider bendsknees and ankles during active riding. A torsion spring acts about thisnon-encoded axis to keep the cuff pressed firmly against the rider. Thespring is not preloaded and generates approximately 20 kg/mm per degree,such that the rider experiences a force of 1.5 kg at his knee in atypical riding posture (25° forward of unloaded position) and 3 kg atfull forward travel (50°). At full forward position there is a stopwhich allows the rider to command pitch torque to the chassis throughforward knee pressure.

Due to variations in the underlying terrain, there are situations inwhich the rider's body position does not necessarily correlate tointended yaw input. One situation is traversing a sideslope, duringwhich time the rider will need to lean uphill to stay balanced. Anothersituation is striking an obstacle with one wheel, which may cause themachine to roll sharply while the rider stays upright. During both ofthese situations the potentiometer will record that the body positionhas moved relative to the machine, which is normally interpreted as ayaw command. While terrain-induced body position presents a challenge toa system which translates body position into yaw, steps can be taken tomitigate these situations. A system discussed below addresses theterrain-induced yaw inputs described above with separate algorithms forside slopes and sudden wheel impacts.

On machines with yaw inputs derived from body position it is necessaryto compensate for the difference in roll angle to the bodies' naturaltendency to line up with gravity. The only exception to this is the casewhere there is a sufficient restoring force on the yaw input to overcomethe rider's natural tendency to keep the yaw input in-line with theirbody.

In order to roll compensate the yaw input a calculation needs to bemade. This calculation entails measuring the amount of roll angle thatcouples into the yaw input. The following function is used to calculatea roll compensated yaw input:

roll compensated yaw input=yawinput−(Gain_RollContributionToYawInput*roll).

For example: Gain_RollContributionToYawInput=(1.44/1.0), where 1 countof roll gives 1.44 counts of yaw.

In accordance with another embodiment of the invention, the rider mayhit a button which resets their current knee position as neutral.

Although there is no measurable backlash in the centering device, themechanism can flex before overcoming the centering device preload. Thistranslates to about 1° of knee motion in either direction which does notcommand a yaw. This can be reduced by increasing the stiffness of thestructure relative to the preload. A loose fit between the knee cuff andthe knee adds an additional 1-2° of motion that does not produce asignal.

Additionally, the potentiometer may exhibit hysteresis, which may becompensated by addition of a software dead band. Dead band has theadvantage of allowing the rider a small amount of motion, which reducesfatigue. However, precision and slalom performance is compromised bydead band. User-adjustable or speed-sensitive deadband may also beembodied.

Asymmetrical gains may be useful to compensate for the asymmetryinherent in measuring the motion of one of two legs. Since body positiondetermines yaw input, appropriate mapping of rider position to lateralacceleration at speed is more significant in this device than in ahand-steered device.

In accordance with another embodiment of the invention, described withreference to FIG. 10C, two steel “whiskers” 930 (approximately 50 cmlong and 35 cm apart), are provided at approximately hip height. Leaningleft or right pushes on the whiskers and twist the potentiometer (gainsare doubled in software). The length of the whiskers is preferred sothat the rider, in the course of leaning backwards and forwards, doesnot exit the device and lose yaw input capability.

Another embodiment of the invention, discussed with reference to FIG.10D, employs two body torso position sensors 940 with handgrips 942bolted to either side of the chassis. Smooth planks 944 (approximately60 cm long and adjustably spaced), are attached to a leaning shaft oneither side of the rider's ribcage to sense body position above thewaist so as to account for body lean accomplished by bending at thewaist. A longitudinal axis of rotation advantageously eliminateslean-sensitive gains that might be present in other designs.

Linear Slide Directional Input

The “linear slide” directional input device is a shear force sensitivemeans of steering a personal transporter. The device has a platform thatcan slide in the lateral direction of the machine, directly in line withlateral accelerations seen during turning.

During a turn the user feels a lateral acceleration in the vehicle frameof reference. The lateral acceleration causes a shear force between theuser and the vehicle, which is reacted through the footplate and thehandlebar. Because the user has two points to react this force, one canbe used as a directional input driven through the other. In thisimplementation the user reacts on the handlebar. The linear slidemechanism measures this reaction through displacement of the platform,and uses it as a directional command. This input method is directlycoupled to lateral acceleration, with the user modulating the couplingby reacting off the handlebar. At zero lateral acceleration, the usercan create a directional input by pushing laterally on the handlebar. Atnon-zero acceleration, the user's handlebar force adds to the lateralacceleration force to create the input.

The linear slide mechanism was designed to sit on top of the chassis ofa personal transporter, replacing the foot mat assembly. It ismarginally smaller that the existing foot plate area to allow for platedisplacement. The platform wraps around the control shaft base. Themaximum platform travel is approx +/−1 inch. The footplate mechanism 950is shown in FIG. 11.

The assembly is clamped to the platform of a human transporter with fourblocks that capture the base plate of the slide. The blocks allow theassembly to move vertically in order to activate the rider detectswitches. Because the weight of the assembly alone is sufficient toactivate the switches, it is counterbalanced with two ball plungers.These insure that the rider detect switches only activate when a rideris on the transporter.

The upper platform rides on ½ inch linear ball bearings which, in turn,ride on a ground rod mounted to the lower platform. A spring and stoparrangement provides preloaded centering force.

A linear potentiometer converts the platform position to an analogvoltage, which is input to the user interface circuit board in place ofthe potentiometer employed in conjunction with the twist grip embodimentdescribed above with reference to FIG. 3.

Algorithms for operation of the linear slide yaw input are essentiallythose of the twist grip yaw input, as described in detail in U.S. Pat.No. 6,789,640, albeit with an opposite yaw gain polarity.

Several embodiments of the invention are related to the device justdescribed. In accordance with one embodiment, individual pivotingfootplates are shear sensitive and pivot at a point above the surface ofthe plate, preferably 4 to 6 inches above the surface. This allows somelateral acceleration coupling, but give the user the ability tostabilize the coupling through leg or ankle rotation.

Alternatively, the linear slide may be moved to the handlebar. Thisallows a user to use his legs for reacting to lateral accelerationswithout commanding input. However, since most of the lateralacceleration is reacted in the legs, coupling with lateral accelerationis largely lost by moving the linear slide to the handlebar.

Speed Limiting

In a further embodiment, any of the foregoing embodiments of a vehiclein accordance with the present invention may be provided with speedlimiting to maintain balance and control, which may otherwise be lost ifthe wheels (or other ground-contacting members) were permitted to reachthe maximum speed of which they are currently capable of being driven.

Speed limiting is accomplished by pitching the vehicle back in thedirection opposite from the current direction of travel, which causesthe vehicle to slow down. (As discussed above, the extent and directionof system lean determine the vehicle's acceleration.) In thisembodiment, the vehicle is pitched back by adding a pitch modificationto the Pitch State Estimator pitch value. Speed limiting occurs wheneverthe vehicle velocity of the vehicle exceeds a threshold that is thedetermined speed limit of the vehicle. The pitch modification isdetermined by looking at the difference between the vehicle velocity andthe determined speed limit, integrated over time.

Alternatively, the balancing margin between a specified maximum poweroutput and the current power output of the motors may be monitored. Inresponse to the balancing margin falling below a specified limit, analarm may be generated to warn the user to reduce the speed of thevehicle. The alarm may be audible, visual, or, alternatively the alarmmay be tactile or may be provided by modulation of the motor drives,providing a ‘rumbling’ ride that is readily perceived by the user.

The automatic pitch modification sequence, in response to a detectedspeed at a specified speed limit, is maintained until the vehicle slowsto the desired dropout speed (some speed slightly below the speedlimit), and then the pitch angle is smoothly returned to its originalvalue.

One method for determining the speed limit of the vehicle is to monitorthe battery voltage, which is then used to estimate the maximum velocitythe vehicle is currently capable of maintaining. Another method is tomeasure the voltages of the battery and the motor and to monitor thedifference between the two; the difference provides an estimate of theamount of velocity margin (or ‘balancing margin’) currently available tothe vehicle.

The described embodiments of the invention are intended to be merelyexemplary and numerous variations and modifications will be apparent tothose skilled in the art. In particular, many of the controllers andmethods of direction and speed control described herein may be appliedadvantageously to personal transporters that are not balancing personaltransporters. Balancing transporters present particular requirements forcombining yaw and balance controls, as discussed in the foregoingdescription and in U.S. Pat. No. 6,789,640. All such variations andmodifications are intended to be within the scope of the presentinvention as defined in any appended claims.

1. A method for alerting a rider of a vehicle to reduce a speed of thevehicle, the method comprising: sensing a specified condition of thevehicle representative of the vehicle speed; and providing a modulatedsignal to a motorized drive of the vehicle to shake the rider inresponse to the specified condition indicating that a speed limit isexceeded.
 2. The method according to claim 1, wherein the specifiedcondition is a current power output of the motorized drive.
 3. Themethod according to claim 2, wherein the speed limit is a specifiedmaximum power output of the motorized drive.
 4. The method according toclaim 1, wherein the modulated signal is maintained until the vehicleslows to a desired dropout speed below the speed limit.
 5. The methodaccording to claim 4, wherein the signal to the motorized drive issmoothed after the dropout speed is reached.
 6. The method according toclaim 1, additionally comprising: monitoring a vehicle battery voltage;estimating a maximum vehicle speed from the battery voltage; and settingthe speed limit based on the maximum vehicle speed.
 7. The methodaccording to claim 6, additionally comprising: monitoring a motorizeddrive voltage; and calculating a velocity margin from the differencebetween the battery voltage and the motorized drive voltage.
 8. Themethod according to claim 7, wherein the modulated signal is provided tothe motorized drive when the velocity margin falls below a specifiedlimit.
 9. The method according to claim 1, wherein the modulated signalis a periodic ripple modulation.
 10. An alarm system for alerting arider of a vehicle having a motorized drive to reduce a speed of thevehicle, the alarm system comprising: a sensor for sensing a specifiedcondition of the vehicle representative of the vehicle speed; and acontroller for providing a modulated signal to the motorized drive toshake the rider in response to the specified condition indicating that aspeed limit is exceeded.
 11. The alarm system according to claim 10,wherein the sensor senses a current power output of the motorized drive.12. The alarm system according to claim 11, wherein the speed limit is aspecified maximum power output of the motorized drive.
 13. The alarmsystem according to claim 10, wherein the controller continues toprovide the modulated signal until the vehicle slows to a desireddropout speed below the speed limit.
 14. The alarm system according toclaim 13, wherein the controller provides the motorized drive with asmoothed when the vehicle speed is below the dropout speed.
 15. Thealarm system according to claim 10, wherein the sensor monitors abattery voltage of the vehicle; and wherein the controller estimates amaximum vehicle speed from the battery voltage and sets the speed limitbased on the maximum vehicle speed.
 16. The alarm system according toclaim 15, additionally comprising at least a second sensor monitoring amotorized drive voltage of the motorized drive; and wherein thecontroller calculates a velocity margin from the difference between thebattery voltage and the motorized drive voltage.
 17. The alarm systemaccording to claim 16, wherein the controller provides the modulatedsignal is to the motorized drive when the velocity margin falls below aspecified limit.
 18. The alarm system according to claim 10, wherein thecontroller provides a modulated signal that is a periodic ripplemodulation.